Cellulose esters having a DS less than 3 (i.e., less than fully substituted) find wide application in commerce. These materials serve many markets such as molding plastics, clear sheets, filter tow, and as coatings polymers, to name a few forms and applications. Methods for their economical and selective preparation are clearly desirable.
Moreover, polymers which have affinity for water are of great commercial interest. Water-absorbent polymers, such as poly(acrylates), are used commercially in diapers, absorbent pads, and similar high-volume consumer goods. Water-soluble polymers also find widespread use in the marketplace. They are used in foods, oil field applications, cosmetics, and pharmaceuticals, to cite a few examples. It is clear, therefore, that new polymer compositions with high affinity for water would have considerable commercial potential. Similarly, new and superior processes for the manufacture of polymers with high water affinity would be of considerable benefit.
It is well known in the art that cellulose acetates with a low degree of substitution have high affinity for water. C. J. Malm (British Patent 356,012 (1929)) disclosed the preparation of cellulose monoacetate (CMA) by the sulfuric acid-catalyzed hydrolysis of cellulose triacetate (CTA) in aqueous sulfuric acid. The product, having a DS of 0.6-0.8 acetyls (DS=number of substituents per anhydroglucose ring), was soluble in water. This necessitated isolation by addition of a nonsolvent. It is difficult to avoid contamination of the CMA from this process by sulfate salts. Other drawbacks of the Malm procedure include the long reaction times and the necessity for continuous or sequential addition of water to maintain reaction rates, resulting in a dilute reaction mixture and difficulties in recovery of by-product acetic acid. Additionally, the sulfuric acid catalyst promotes rapid degradation of the molecular weight of the polymer.
Similar work by C. L. Crane (U.S. Pat. No. 2,327,770 (1943)) disclosed that cellulose diacetate could be hydrolyzed in aqueous acetone or aqueous alcohol using sulfuric acid catalyst to afford a water-soluble CMA. This process suffers shortcomings which are similar to those of the Malm process described above.
In U.S. Pat. No. 2,005,383, T. F. Murray and C. J. Staud disclosed the use of zinc iodide in ethanol to solvolyze cellulose triacetate (CTA). This process afforded a product with DS about 1.75, required long reaction times, and consumed large amounts of zinc iodide (10 parts ZnI per part CTA). Even with this amount of zinc iodide, 40 hours reaction time was required to produce the product having a DS of only 1.75.
U.S. Pat. No. 2,801,239 (1957, G. D. Hiatt, L. W. Blanchard, Jr., and L. J. Tanghe) teaches the use of zinc chloride as a catalyst for the acetylation of cellulose. The inventors state that the zinc chloride must be removed before the hydrolysis of the resulting ester because the zinc chloride limits the amount of water which may be used in the hydrolysis and increases the rate at which the viscosity (which is indicative of molecular weight) is reduced. This result would lead one to expect that Lewis acid metal salts would be undesirable catalysts for the solvolysis of cellulose esters.
In U.S. Pat. No. 2,836,590 (1958) H. W. Turner discloses high temperature (&gt;180.degree. C.) alcoholysis of cellulose acetate without the use of catalysts. At the temperatures disclosed by Turner, cleavage of the 1,4-glycosidic linkages of the cellulose ester backbone competes with the desired deacylation.
A different approach to CMA is disclosed by M. Diamantoglou, A. Brandner, and G. Mayer in U.S. Pat. No. 4,543,409 (1985). They acetylated cellulose in homogeneous solution (in N,N-dimethylacetamide (DMAC) containing lithium chloride). The product was a cellulose monoacetate as indicated by its low DS, but was not soluble in water. There are serious environmental and economic concerns associated with the use of the toxic and expensive DMAC as a commercial reaction solvent. It is believed in the art that the two basic requirements for water solubility are that (i) the DS be in the range of 0.5-1.1 and that the relative degree of substitution (RDS) at the three possible sites of substitution be roughly equal. Currently, only the method taught by Malm fulfills these requirements (Shibata et al., J. Poly. Sci., Poly. Chem. Ed. 1985, 23, 1373; Kamide et al. Polym. J. 1987, 19, 1405). There is, therefore, a need in the art for a process to prepare cellulose acetates with a low degree of substitution, and possessing high affinity for water. Such a process will desirably use solvents which are inexpensive and easily recycled. Such a process will also desirably employ catalysts which are either powerful enough to be used in small amounts or inexpensive enough to be used in large amounts when necessary. A desirable process will allow for easy and economical product isolation, and simple and economical recycle of solvents. Also desirable is a process which requires economically short reaction times, is reliable and repeatable, and uses commercially practical reaction temperatures. The combination of catalyst and reaction conditions will desirably be such that the molecular weight of the product polymer is not severely degraded.